The present invention relates to a compact heating furnace for drawing a long glass rod, which can be effectively used for drawing base material of optical fiber.
Conventionally, such a heating furnace as shown in FIG. 1 has been used for drawing a glass rod, for example, base material of optical fiber. This known heating furnace has a furnace body 21. A cylindrical inlet opening 22 for inserting base material A of optical fiber into the furnace body 21 is provided at a central portion of a top wall of the furnace body 21, while a cylindrical outlet opening 23 for dragging a drawn glass portion D out of the furnace body 21 is provided at a central portion of a bottom wall of the furnace body 21. A feed bar E for feeding the base material A to a heating element 26 in the furnace body 21 and a drag bar C for dragging the drawn glass portion D out of the furnace body 21 are, respectively, coupled with upper and lower ends of the base material A of optical fiber beforehand. A sealing cover 24, which is formed with a bore having such a dimension as to seal a clearance between the feed bar E and the bore, is hermetically placed on an upper end of the inlet opening 22. Meanwhile, supply ports 28 and 29 for supplying inert gas into the furnace body 21 are, respectively, provided at the inlet opening 22 and the outlet opening 23.
In the case where the base material A of optical fiber is drawn by using the known heating furnace of FIG. 1, a blind cover 24a is initially placed on the inlet opening 22. Then, in an open state of a gas exhaust vent 30 of a lower cover 25, inert gas is supplied into the furnace body 21 from the supply ports 28 and 29 by a supply device (not shown) for supplying inert gas so as to occupy atmosphere in the furnace body 21 such that excessive inert gas flows out of the gas exhaust vent 30. Subsequently, the heating element 26 in the furnace body 21 is heated to a predetermined temperature for drawing the base material A of optical fiber so as to be maintained at the predetermined temperature. Thereafter, in order to insert the base material A of optical fiber into the furnace body 21, the gas exhaust vent 30 of the lower cover 25 is closed. Then, after the blind cover 24a has been removed from the inlet opening 22, the base material A of optical fiber is inserted into the furnace body 21 and a lower end of the base material A is heated by the heating element 26. Subsequently, when the lower end of the base material A has been heated so as to be softened, the drag bar C is dragged downwardly so as to form the drawn glass portion D. Since the base material A of optical fiber is held at high temperatures during and after heating of the base material A, it is necessary to maintain atmosphere of inert gas in the furnace body 21.
However, when long base material of optical fiber is drawn by using the known heating furnace of FIG. 1, an upper end of the base material projects out of the inlet opening 22 provided at the top wall of the furnace body 21 and thus, it becomes impossible to hermetically seal the upper end of the inlet opening 22 by the sealing cover 24. Therefore, the cylindrical inlet opening 22 of the heating furnace is required to have a length necessary for accommodating therein the base material of optical fiber. Meanwhile, the feed bar E is also required to have a length sufficient for feeding the base material of optical fiber from the upper end of the inlet opening 22 towards the heating element 26, thereby resulting in difficult operation. Thus, the heating furnace as a whole becomes larger in height, thereby resulting in deterioration of its space factor. Furthermore, the known heating furnace has such a drawback that the long feed bar is required to be used, thus resulting in extreme deterioration of its working efficiency.